This invention relates generally to flat panel image display devices utilizing line cathodes and particularly to systems for compensating for electron beam current variations caused by the mechanical vibration of the cathodes in such systems. Exemplary of systems which utilize line cathodes are the two systems described in U.S. Pat. No. 4,121,137, issued to Thomas L. Credelle and U.S. Pat. No. 4,126,814, issued to Frank J. Marlowe. The systems described in these two patents relate to flat panel display devices and each of the patents describes a system for maintaining a uniform brightness across the entire viewing area of such a display.
U.S. Pat. No. 4,126,814 is of particular interest because, as will be fully described hereinafter, the inventive features of the instant invention can be combined with the unique circuitry described in the patent to realize an improved visual display in flat panel display devices.
The environment of the instant invention can be understood by making reference to FIG. 1 which shows an exemplary flat panel display device of the type presently known in the art. In order to show the internal structure the upper and lower portions of FIG. 1 are shown in partial cut-away section.
In FIG. 1 the flat panel display device is generally indicated by reference numeral 10 and includes a back panel 11 and a display panel 12 which are coupled by two side walls 13 and upper and lower walls 14 and 16, respectively. The envelope 10, formed by the two planar panels and four sides, is evacuated in the same manner as other cathodoluminescent display devices. A plurality of nonconductive vanes 17 divide the envelope 10 into a plurality of electron beam channels 18. Each of the channels 13 contains two grids or meshes 19 and 21 which are parallel spaced along the length of the channels 18 to form guides so that electron beams can travel the length of the channels 18 between the two meshes 19 and 21. The construction and operation of the display device shown in FIG. 1 are fully described in U.S. Pat. Nos. 4,121,137 and 4,126,814 and these descriptions are incorporated by reference herein.
As shown in FIG. 1, cathode 22 is supported at both ends by mounting means 23 and 24 so that the entire length of the cathode extending between the two mounts 23 and 24 is free. One end of the cathode 22 is provided with an electrical connecting means 26 permitting the application of an energizing voltage to the cathode. The vanes 17 divide the envelope into a plurality of equally dimensioned channels (for example 40) and, therefore, the cathode 22 can be viewed as a plurality of cathodes equal in number to the number of channels and individually placed at the centers of the channels 18.
The display device 10, also includes a collector 25 extending the entire length of the side 14 and transversely across the ends of all of the channels 18. Electron beams propagating along the channels 18 and not ejected onto the display surface 12 are made to impact with the collector 25. As described hereinafter the currents resulting from such impacts are detected at the collector 25 and used to establish voltages. These voltages vary because of vibration of the cathode and thus are used as vibration compensation voltages to offset the detrimental effects of the cathode vibration on the display panel 12.
Cathodoluminescent display devices of the type described with respect to FIG. 1 have been demonstrated as being feasible for large flat panel displays. However, because a single line cathode is used and because the cathode is supported only at the two ends, the cathode is placed under tension in order to accurately position and space the cathode with respect to the two meshes 19 and 21. The cathode therefore is subjected to vibration which degrades the fidelity of the display because the electron beam current fluctuates as a result of the vibration. A more complete understanding of the effect of the vibration on electron beam current can be gained by viewing FIGS. 2 and 3.
In FIG. 2 horizontal displacement in the Z direction results in a change .DELTA.z of the spacing z between the cathode 22 and the guide meshes 19 and 21. This motion in FIG. 1 appears as vertical motion. Vibration in the Y direction changes the position of the cathode 22 with respect to the center of the space between the meshes 19 and 21; in FIG. 1 such motion is horizontal. Vibration of the cathode 22 can occur in the Z direction, the Y direction or the vector summation of any direction between those two directions. The effect of vibration in the Y direction on electron beam current is different from the effect of vibration in the Z direction.
Typically the cathode 22 is positioned along the center line of the spacing between the guide meshes 19 and 21. With the cathode in this position electrons emitted by the cathode 22 enter the spacing between the guide meshes 19 and 21 substantially uniformly dispersed above and below the center line. However, because vibration in the Y direction vertically displaces the cathode 22 in FIG. 2, the balance of electron entrance with respect to the center line is upset so that fewer electrons enter the beam guide when the cathode is displaced from the center position. The instant invention does not provide specific means for compensating for this vibration; however, some compensation is automatically provided because Y direction vibration causes a decrease in electron beam current. Accordingly, because the inventive system provides compensating voltages which are related to increases and decreases in the electron beam current of each electron gun of the system, some compensation for Y directed vibration is automatically provided.
Vibration in the Z direction causes the spacing z between the cathode 22 and the grid meshes 19 and 21 to increase and decrease at a frequency which is identical to the vibration frequency of the cathode. As is known to those skilled in the art, the frequency of vibration of a wire under tension supported only at the ends is inversely proportional to the length of the wire and the square root of the ratio of the mass per unit length of the wire and the tension in the wire. Because all of these factors are known, it is possible to determine the resonant frequency of the cathode. Typically, this frequency will be between 20 Hz and 200 Hz. The frequency of the application of the vibration compensating voltages will be dependent upon the horizontal sweep time of the electron beams, typically this is 15 KHz. Because of the substantial difference, between these two frequencies (which in all instances is at least 75:1) the cathode can be considered to be at rest during the period of horizontal sweep and for the purposes of applying the compensation voltages and for determining the magnitude of such voltages.
The effect of cathode vibration in the Z direction can be expressed as: ##EQU1## where
i=beam current
I=quiescent current at z=z.sub.o
z=z.sub.o +.DELTA.z instaneous guide cathode separation
z.sub.o =z with no cathode vibration
V.sub.m =modulation potential
V.sub.c =cutoff voltage
By denoting the total compensation voltage required as V, the compensation voltage is described by: ##EQU2## where
V=total compensation voltage
V.sub.o =quiescent compensation voltage
V.sub.1 =first harmonic compensation voltage
V.sub.2 =second harmonic compensation voltage
V.sub.i =highest harmonic for which a compensation voltage is required
When V=V.sub.o and z=z.sub.o there is no vibration and no compensation is required. However, when z is not equal to z.sub.o the necessary compensation can be shown to be: ##EQU3##
If only the fundamental is compensated for, it can be mathematically shown that the required compensation voltage defined by expression (3) reduces to ##EQU4## where C is a constant.
It is evident from equation 4 above that the instantaneous electron beam current is linearly related to the displacement .DELTA.z of the cathode with respect to the opening between the guide meshes 19 and 21. However, because the cathode 22 displays the characteristics of a vibrating wire, the deplacement .DELTA.z is different for each of the electron beam channels 18. This can be understood by making reference to FIG. 3.
In FIG. 3 the cathode 22 is shown in a rest position 22a and an instantaneous displaced position 22b, under conditions appropriate to equation 4. Additionally, because the cathode is centered with respect to the display panel 10, the maximum displacement occurs between the 20th and 21st modules, for the 40 module example given herein. It naturally follows that the minimum displacement occurs at the center of the 1st and 40th modules. In FIG. 3, five of the forty modules are indicated as n.sub.1, n.sub.10, n.sub.20, n.sub.31 and n.sub.40. In FIG. 3, the maximum displacement between the 20th and 21st modules, is represented by the amplitude A. The displacement at any arbitrary position along the cathode is represented by .DELTA.z. From the definition of the configuration of a vibrating wire, .DELTA.z can be shown to be: ##EQU5## where:
.DELTA.z=amplitude at right hand boundary of module "n"
N=the total number of modules
s=distance between outside boundaries of outermost modules and the cathode mounts
M=the center to center distance between adjacent modules
A=.DELTA.z at the antinode
n=the module number along the line cathode length.
Equation (5) and inspection of FIG. 3 show that the displacement .DELTA.z is symmetrical about the boundary between the 20th and 21st modules which is located at the antinode of the vibrating cathode. Hence, the displacement .DELTA.z at the right boundary for the 10th and 30th modules is the same, as is the displacement 5th and 35th modules, etc.
In FIG. 3 the cathode 22 is supported at a distance s from the left and right edges respectively of the outermost modules n1 and n40. For this reason the displacement .DELTA.z for the two outer electron guns is always greater than zero when the cathode 22 is vibrating. The two portions s of the cathode 22 are not associated with any of the channels 18 and thus have no affect on the quality of the visual display. Also, the antinode is located at the boundary between the modules n20 and n21. The amplitude .DELTA.z given by equation 5, therefore, is not located at the center of the modules, but rather is defined at points located along the right hand boundary of each module. However, because the current variations which occur due to vibration of the cathode are sensed at an electron gun associated with the center of module n20, the sensing occurs at a location slightly displaced from the antinode. This displacement would result in the introduction of a slight error because the distance .DELTA.z at the center of module n20 is less than the amplitude A at the antinode. Equation 5 can be corrected for this displacement by subtracting the term: "M/2" from the numerator, where M represents the center to center spacing of adjacent modules. In equation 5 the denominator is equivalent to the length of the cathode 22 between the mounts 23 and 24. The amplitude at the center of the "nth" module is therefore: ##EQU6## where:
.DELTA.z=displacement along the cathode at center of module "n"
A=displacement at the antinode
N=the total number of modules (electron guns)
M=the center to center distance of adjacent modules
s=the distance between the outside surfaces of the outermost modules and the cathode mounts
n=the module number.
The above description has not considered the possible effects of the harmonic frequencies above the fundamental of the vibration of the cathode. Typically, higher harmonic frequency effects on the beam current will not be sufficient to degrade the visual display and therefore no additional compensation ordinarily is required. Should such additional compensation be required, it will be recognized that the second harmonic frequency will be twice that of the fundamental. The nodes will appear at the supported ends and between modules n20 and n21. One of the antinodes will appear between modules n10 and n11 and the other antinode between modules n30 and n31.